The present invention relates to developing apparatuses for developing an electrostatic latent image formed on an image bearing member. In particular, it relates to such a developing apparatus that is employed in image forming apparatuses, for example, copying machines or printers, which employ an electrophotographic recording system, an electrostatic recording system, or the like.
In an electrophotographic image forming apparatus or the like, an electrostatic latent image is formed on the image bearing member, and the latent image is developed with the use of a developing apparatus and developer.
A developing apparatus comprises a developer carrying member which delivers developer close to the peripheral surface of the image bearing member by carrying developer on its peripheral surface. The developer carried close to the peripheral surface of the image bearing member develops an electrostatic latent image on the image bearing member, or visualizes the latent image, as an alternating electric field is formed between the developer carrying member and the image bearing member. Generally, the image bearing member and the developer carrying member are constituted of a photosensitive drum and a development sleeve, respectively.
There are several methods for developing a latent image. One of the well known methods is the method which employs two component developer composed of carrier and toner. According to this method, a magnetic brush is formed of this developer by the magnet encased in a development sleeve, on the peripheral surface of the development sleeve which is disposed in such a manner that a microscopic gap is maintained between the peripheral surfaces of the development sleeve and the image bearing member. A latent image formed on the image bearing member is developed as the magnetic brush brushes, or virtually touches, the peripheral surface of the image bearing member. As for the alternating electric field formed between the development sleeve and the photosensitive drum, there are two types: an alternating electric field (Japanese Laid-Open Patent Application Nos. 34060/1980 and 165082/1984) and a DC electric field.
A development apparatus based on a magnetic brush formed of two component developer is generally structured as depicted in FIG. 4. In the drawing, a referential figure 105 designates a developer container of a developing apparatus which relies on a magnetic brush form of two component developer. The developer container 105 contains two component developer 103, that is, a mixture of nonmagnetic toner and magnetic carrier. The developer container 105 has a wide opening which faces a photosensitive drum 100. In this wide opening, a development sleeve 109 is disposed, and in the development sleeve, a magnetic roller is nonrotatively disposed. The positional relationship between the development sleeve 109 and photosensitive drum 100 is such that a predetermined gap is always maintained between the peripheral surfaces of the development sleeve 109 and the photosensitive drum 100. Above the development sleeve 109, a regulator blade 111 is attached to the developer container 105. This blade 111 regulates the thickness of the developer layer formed on the peripheral surface of the development sleeve 109. Approximately the bottom half of the internal space of the developer container 105 is partitioned into a developer space R1 and a stirring space R2 by a partition wall 113. Above the stirring space R2, a toner storage space R2 is located, which stores the replenishment toner 119.
The development sleeve 109 is rotated in such a direction that the peripheral surfaces of the development sleeve 109 and the photosensitive drum 100 move in the same direction at the location where the two surfaces face each other. As the development sleeve 109 is rotated, the two component developer 103 in the developer container 105 is adhered to the peripheral surface of the development sleeve 109 by the effect of the magnetic roller 107, and then, is carried to the location at which the peripheral surfaces of the development sleeve 109 and the photosensitive drum 100 face each other. As stated above, there is provided a gap 125 between the peripheral surfaces of the development sleeve 109 and the photosensitive drum 100 so that the developer on the development sleeve 109 develops the electrostatic latent image on the peripheral surface of the photosensitive drum 100 as the developer makes contact with the peripheral surface of the photosensitive drum 100.
The toner density of the two component developer 103 within the developer container 105, that is, the ratio (T/C ratio) of the amount of the toner relative to the amount of the carrier in the developer, is kept constant by allowing the toner in the toner storage space R3 to free fall through a replenishment hole in the bottom wall of the toner storage space R3, by the same amount as the amount of toner consumed through development activity.
As for the method for detecting and/or maintaining the T/C ratio of the developer within the developer container 105, various methods have been proposed, and some of them have been put to practical use. For example, according to one of the methods, an optical density sensor is disposed adjacent to the photosensitive drum 100. In obtaining the T/C ratio, light is projected upon a toner image formed on the peripheral surface of the photosensitive drum 100 by developing the latent image formed on the peripheral surface of the photosensitive drum 100, and the amount of the light which is transmitted through, or is reflected by, the tuner image is detected by the optical density sensor. Then, the T/C ratio is determined from the detected amount of the light. The amount of the toner allowed to free fall is adjusted according to the thus obtained T/C ratio, to keep the T/C ratio of the developer constant. According to another method, an optical density sensor is disposed on the development sleeve 109. In obtaining the T/C ratio, light is projected upon the developer layer on the peripheral surface of the development sleeve 109, and the T/C ratio is determined from the amount of the light reflected by the developer layer. Then, the amount of the toner to be added to the developer is adjusted according to the thus obtained T/C ratio of the developer.
However, the former method that maintains the T/C ratio of the developer in the developer container 105 on the basis of the toner density of the toner image suffers from the problem that it is very difficult to find a space for the sensor in a developing apparatus since the size of copying machines and image forming apparatuses has been continuously reduced in recent years. The latter method, that is, the method that maintains the T/C ratio in the developer container 105 on the basis of the detected T/C ratio of the toner layer on the development sleeve 109, also suffers from a problem, that is, the problem that the sensor is contaminated due to the scattering of the toner particles, failing to accurately detect the T/C ratio of the developer layer on the peripheral surface of the development sleeve 109.
Fortunately, there is another method for maintaining the T/C ratio of the developer in the developer container 105. According to this third method, a sensor of a coil type is disposed as the toner density sensor in the developer container 105, to obtain the apparent permeability of a specific volume of the developer; the inductance of the coil of the sensor is measured. Then, the amount of the toner to be added is adjusted on the basis of the thus obtained T/C ratio of the specific volume of the developer adjacent to the coil type sensor.
More specifically, according to this method that employs a toner density sensor of a coil type, the increase in the permeability of a specific volume of the developer means the decrease in the T/C ratio of the specific volume of the developer, that is, the decrease in the toner density of the developer. Therefore, as the permeability of the specific amount of the developer increases, toner replenishment is started. On the other hand, the decrease in the permeability of the specific volume of the developer means the increase in the T/C ratio of the specific volume of the developer, that is, the increase in the toner density of the developer, and therefore, as the permeability of the specific volume of the developer decreases, toner replenishment is stopped.
The toner density sensor of a coil type employed in this third method is inexpensive in unit cost, does not create the aforementioned problem pertaining to the installation space, or the sensor contamination by the scattering of the toner particles. Therefore, a T/C ratio detecting means employing a coil type toner density sensor, and a toner density controlling apparatus employing such a T/C ratio detecting means, are best for an inexpensive copying machine or image forming apparatus, which has only a small internal space.
Yet, even a coil type toner density sensor has its own problem. That is, if the bulk density of the developer in the developer container 105 is affected for one reason or another, the apparent permeability of the developer also affected. Therefore, the sensor output may change even if the T/C ratio of the developer In the developer container 105 remains virtually the same. In other words, such a situation may occur that a signal which indicates the decrease in toner density is outputted, starting toner replenishment, even when the toner density of the developer in the developer container 105 has not decreased, or that the signal which indicates the decreases in the toner density is not being outputted, fails to start the toner replenishment, in spite of the fact that the toner density has decreased.
In the case of the first scenario, an excessive amount of toner is supplied, causing the following problems. For example, the excessive amount of replenished toner renders image density undesirably high, causes the developer to overflow from the developer container 105, and/or renders the toner ratio of the developer in the developer container 105 excessively high, preventing the toner from being charged to the desirable potential level, which in turns allows the toner particles to scatter. In the case of the latter scenario, the amount of the toner in the developer in the developer container 105 decreases, causing the following problems. For example, the decrease in the amount of toner renders image quality inferior, renders the image density low, or causes the toner to be charged to an undesirably high potential level, which in turn renders the image density to be undesirably low.
Thus, the inventors of the present invention studied the causes of the change in the bulk density of the developer in the developer container 105, and discovered the following.
First, the bulk density of the developer is greatly affected by the particle shape of the developer. The particles of the toner produced by pulverization or the like are irregular in shape; they are different from each other in shape. Therefore, the bulk density of the developer containing such toner is liable to change depending on whether the developer is in a static state or is flowing, and whether a developing apparatus containing such developer is operational or not. Further, the shape of the toner particle produced by pulverization, in a batch of developer is liable to change as the batch of developer is in use for a substantial length of time. Among the types of the bulk density change described above, the one attributable to the toner particle shape change which occurs while the developer is in use for a substantial period of time is the largest.
The second cause for the bulk density change is the compression of the developer. More specifically, as the development sleeve 109 is rotated in the same direction as the photosensitive drum 100, and the thickness of the layer of the developer coated on the peripheral surface of the development sleeve 109 is regulated by the regulator blade, a certain amount of the developer is trapped adjacent to the regulator blade 111, being thereby compressed. Further, in order to cause the two component developer in the developer container 105 to be borne on the development sleeve 109 by the magnetic force, the magnetic roller 107 must be such that the magnetic force at the magnetic poles N3 and S2 must be powerful enough to pick up a sufficient amount of the developer. Such a magnetic roller magnetically generates a strong magnetic field between the development sleeve 109 and the regulator blade 111, and the trapped developer is gradually compressed, magnetically as well as mechanically, causing the shape of the toner particles in the developer to change, or the additive particles to be compressed into developer particles. As a result, the bulk density of the developer changes.
Thirdly, the bulk density of the developer is affected by the amount of charge each toner particle carries. That is, in the case of a developing apparatus in which the peripheral surface of the development sleeve is rotated in the same direction as the peripheral surface of photosensitive drum at the location where the two surfaces become closest to each other, a certain amount of the developer is trapped and compressed, increasing the developer density in this area, as described above, which in turn increases the amount of triboelectrical charge given to the toner particles. In other words, the amount of charge toner particles receive increases substantially in proportion to the number of the developer sleeve rotations to which each toner particle is subjected. Obviously, the larger the amount of electrical charge each toner particle carries, the larger the repulsive force between the adjacent two toner particles, and therefore, the greater the distance between the adjacent two toner particles becomes, that is, the smaller the bulk density of the developer becomes.
The primary object of the present invention is to provide a developing apparatus in which toner density is detected on the basis of change in the permeability of developer.
Another object of the present invention is to provide a developing apparatus in which change in the bulk density of developer is small.
Another object of the present invention is to provide a developing apparatus comprising: a developer container for containing developer composed of magnetic carrier, and nonmagnetic toner manufactured by polymerization; a developer carrying member, which is disposed in the opening of the developer container to carry the developer; and means for detecting the toner density of the developer, which is constituted of a coil type sensor, and detects the toner density of the developer on the basis of the permeability of the developer.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.